Is it possible to simulate an adjustable lamp's limbs using armatures/bones?
@sambler provided a link to a tutorial by Jonathan Williamson of Blender Cookie. (Thankyou)   The part where the bucket connects to the rest of the arm contains similar mechanics to what I'm looking for.  By following that part of the tutorial I've used the 'track to' constraint and I think this will work.

http://youtu.be/r-V8ihRUsTE

Screencast keys have been removed from blender 2.73 (rc at least)

So I used edit mode to extrude one extra small bone.  ctrl-d (copy) to copy the large bone (in edit mode so it's part of the same group of bones)   then ctrl-shift-c to bring up the menu where I choose 'track to'    then in the bone constraints menu change the head/tail value to 1.